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Abdullah Zuhair Alyouzbaki 18/11/2015 Physiology of nutrition Nutrients in the diet can be classified into macronutrients and micronutrients: 1.macronutrients which are eaten in relatively large amounts to provide fuel for energy. 2.micronutrients (e.g. vitamins and minerals), which do not contribute to energy balance but are required in small amounts because they are not synthesised in the body. Physiology of nutrition Energy balance: energy intake = energy expenditure. Determinants of energy balance. Physiology of nutrition Physiology of nutrition Energy intake is determined by the ‘macronutrient’ content of food. Carbohydrates, fat, protein and (alcohol) provide fuel for oxidation in the mitochondria to generate energy (as adenosine triphosphate (ATP). The energy provided by each of these elements differs: • carbohydrates (16 kJ/g) • fat (37 kJ/g) • protein (17 kJ/g) • alcohol (29 kJ/g). Regulation of energy balance Macronutrients (energy-yielding nutrients) Carbohydrates The ‘available’ carbohydrates (starches and sugars) supply over half the energy in a normal, well-balanced diet. No individual carbohydrate is an essential nutrient, as carbohydrates can be synthesised from glycerol or protein. However, if the available carbohydrate intake is less than 100 g per day, increased lipolysis leads to ketosis. Macronutrients:Carbohydrates Dietary guidelines do not restrict the intake of intrinsic sugars in fruit and vegetables or the sugars in milk. However, intake of extrinsic sugars (sucrose, maltose, fructose), which increase the risk of dental caries and diabetes mellitus, should be limited. Macronutrients:Carbohydrates Individuals who do not produce lactase (‘lactose- intolerant’) are advised to avoid or limit dairy products and foods with added lactose. Starches in cereal foods, root foods and legumes provide the largest proportion of energy in most diets around the world. Macronutrients:Carbohydrates Glycaemic index of foods: This is the area under the curve of the rise in blood glucose concentration in the 2 hours following ingestion of 50 g carbohydrate, expressed as a percentage of the response to 50 g anhydrous glucose. There is emerging evidence linking high glycaemic index foods with obesity and type 2 diabetes. Glycaemic index Macronutrients: Carbohydrates some starches are digested promptly by salivary and then pancreatic amylase, producing rapid delivery of glucose to the blood , other starches are digested more slowly. Sugar alcohols (e.g. sorbitol) that are used as replacement sweeteners can cause diarrhoea if eaten in large amounts. Macronutrients : Carbohydrates Dietary fibre: Dietary fibre is plant food that is not digested by human enzymes in the gastrointestinal tract. Most dietary fibre is known as the ‘non-starch polysaccharides’ (NSP). Dietary fibre can be broken down by the resident bacteria in the colon to produce short-chain fatty acids. This is essential fuel for the enterocytes and contributes to bowel health. The extent of flatus formed is dependent on the food source. Macronutrients : Carbohydrates Dietary fibre: Some types of NSP, notably the hemicellulose of wheat, increase the water-holding capacity of colonic contents and the bulk of faeces. They relieve simple constipation, appear to prevent diverticulosis and may reduce the risk of cancer of the colon. Other viscous, indigestible polysaccharides like pectin and guar gum are important in the upper gastrointestinal tract, where they slow gastric emptying, contribute to satiety, and reduce bile salt absorption and hence plasma cholesterol concentration. Macronutrients : Fats Macronutrients : Fats Fat has the highest energy density of the macronutrients (37 kJ/g) . The principal polyunsaturated fatty acid (PUFA) in plant seed oils is linoleic acid (18 : 2 ω6). This and alphalinolenic acid (18 : 3 ω3) are the ‘essential’ fatty acids, which humans cannot synthesise de novo. They undergo further desaturation and elongation, to produce, for example, γ-linolenic acid (18 : 3 ω6) and arachidonic acid (20 : 4 ω6). These are precursors of prostaglandins and eicosanoids, and form part of the structure of lipid membranes in all cells. Macronutrients : Fats Fish oils are rich in ω3 PUFA which promote the anti- inflammatory cascade of prostaglandin production . They inhibit thrombosis by competitively antagonising thromboxane A2 formation. Substituting saturated fat (i.e. from animal sources: butter) with PUFA in the diet can lower the concentration of circulating low-density lipoprotein (LDL) cholesterol and may help prevent coronary heart disease. Macronutrients : Fats High intakes of trans fatty acids (TFA) reflect the use of oils that have been partially hydrogenated in the food industry. It is recommended that TFAs are limited to < 2% of the dietary fat intake, as they are associated with cardiovascular disease. Cholesterol is also an important substrate for steroid and sterol synthesis, but not an important source of energy. Macronutrients :Proteins Proteins are made up of some 20 different amino acids, of which nine are ‘essential’ i.e. they cannot be synthesised in humans but are required for synthesis of important proteins. Another group of five amino acids are termed ‘conditionally essential’, meaning that they can be synthesised from other amino acids, provided there is an adequate dietary supply. Macronutrients :Proteins The nutritive or ‘biological’ value of different proteins depends on the relative proportions of essential amino acids they contain. Proteins of animal origin, particularly from eggs, milk and meat, are generally of higher biological value than proteins of vegetable origin, which are low in one or more of the essential amino acids. Dietary recommendations for macronutrients Anthropometric measurements Body mass index (BMI) is useful for categorising underand over-nutrition. It is the weight in kilograms divided by the height in metres, squared. For example, an adult weighing 70 kg with a height of 1.75 m has a BMI of 70/1.752 = 22.9 kg/m2. Anthropometric measurements Anthropometric measurements Anthropometric measurements An indication of the degree of abdominal obesity is the waist circumference, measured at the level of the umbilicus. Hip circumference can be measured at the level of the greater trochanters. waist : hip ratios show whether the distribution of fat is android or gynoid . Anthropometric measurements Anthropometric measurements Skinfold measurements can be used to calculate body fat content, whereas relative loss of muscle and subcutaneous fat can be estimated by measuring midarm circumference (at the middle of the humerus) and skinfold thickness over the triceps (using special callipers); muscle mass is estimated by subtracting triceps skinfold thickness from mid-arm circumference. Anthropometric measurements DISORDERS OF ALTERED ENERGY BALANCE Obesity: Obesity is widely regarded as a pandemic, with potentially disastrous consequences for human health. Over one-quarter (25%) of adults in the UK were obese (i.e. BMI ≥ 30 kg/m2) in 2010, compared with 7% prevalence in 1980 and 16% in 1995. Moreover, almost two-thirds of the UK adult population are overweight (BMI ≥ 25 kg/m2), although there is considerable regional and age group variation. Complications of obesity Complications of obesity Complications of obesity it is clear that the lowest mortality rates are seen in Europeans in the BMI range 18.5–24 kg/m2 (and at lower BMI in Asians). It is suggested that obesity at age 40 years can reduce life expectancy by up to 7 years for non-smokers and by 13 years for smokers. Coronary heart disease is the major cause of death but cancer rates are also increased in the overweight, especially colorectal cancer in males and cancer of the gallbladder, biliary tract, breast, endometrium and cervix in females. Complications of obesity Obesity has little effect on life expectancy above 70 years of age, but the obese do spend a greater proportion of their active life disabled. Epidemic obesity has been accompanied by an epidemic of type 2 diabetes and osteoarthritis particularly of the knee. Although an increased body size results in greater bone density through increased mechanical stress, it is not certain whether this translates to a lower incidence of osteoporotic fractures Complications of obesity Obesity may have profound psychological consequences, compounded by stigmatisation of the obese in many societies. Complications of obesity Body fat distribution For some complications of obesity, the distribution rather than the absolute amount of excess adipose tissue appears to be important. Increased intra-abdominal fat causes ‘central’ (‘abdominal’, ‘visceral’, ‘android’ or ‘apple-shaped’) obesity, which contrasts with subcutaneous fat accumulation causing ‘generalised’ (‘gynoid’ or ‘pear-shaped’) obesity; the former is more common in men and is more closely associated with type 2 diabetes, the metabolic syndrome and cardiovascular disease. Body fat distribution The key difference between these depots of fat may lie in their vascular anatomy, with intra-abdominal fat draining into the portal vein and thence directly to the liver. Thus many factors that are released from adipose tissue (including free fatty acids; ‘adipokines’, such as tumour necrosis factor-α, adiponectin and resistin; and steroid hormones) may be at higher concentration in the liver and hence induce insulin resistance and promote type 2 diabetes. Body fat distribution Recent research has also highlighted the importance of fat deposition within specific organs, especially the liver, as an important determinant of metabolic risk in the obese. Aetiology Accumulation of fat results from a discrepancy between energy consumption and energy expenditure that is too large to be defended by the hypothalamic regulation of BMR. A continuous small daily positive energy balance of only 0.2–0.8 MJ (50–200 kcal; < 10% of intake) would lead to weight gain of 2–20 kg over a period of 4–10 years. Given the cumulative effects of subtle energy excess, body fat content shows ‘tracking’ with age such that obese children usually become obese adults. Aetiology The pandemic of obesity reflects changes in both energy intake and energy expenditure ,although both are difficult to measure reliably. The estimated average global daily supply of food energy per person increased from approximately 9.8 MJ (2350 kcal) in the 1960s to approximately 11.7 MJ (2800 kcal) in the 1990s. Portion sizes, particularly of energy dense foods such as drinks with highly refined sugar content and salty snacks, have increased. Aetiology Obesity is correlated positively with the number of hours spent watching television, and inversely with levels of physical activity (e.g. stair climbing). It is suggested that minor activities such as chewing gum may contribute to energy expenditure and protect against obesity. Aetiology Susceptibility to obesity Susceptibility to obesity and its adverse consequences undoubtedly varies between individuals. It is not true that obese subjects have a ‘slow metabolism’, since their BMR is higher than that of lean subjects. Twin and adoption studies confirm a genetic influence on obesity. The pattern of inheritance suggests a polygenic disorder. some of these genes encode proteins known to be involved in the control of appetite or metabolism and some of which have unknown function. However, these genes account for less than 5% of the variation in body weight. Susceptibility to obesity A few rare genetic conditions in which obesity is a feature include the Prader–Willi and Lawrence– Moon–Biedl syndromes. Reversible causes of obesity and weight gain Clinical assessment and investigations In assessing an individual presenting with obesity, the aims are to: • quantify the problem • exclude an underlying cause • identify complications • reach a management plan. Clinical assessment and investigations Severity of obesity can be quantified using the BMI . A waist circumference of > 102 cm in men or > 88 cm in women indicates that the risk of metabolic and cardiovascular complications of obesity is high. Clinical assessment and investigations A dietary history may be helpful in guiding dietary advice. It is important to consider ‘pathological’ eating behaviour (such as binge eating, nocturnal eating or bulimia), which may be the most important issue to address in some patients. Alcohol is an important source of energy intake and should be considered in detail. The history of weight gain may help diagnose underlying causes. Clinical assessment and investigations A patient who has recently gained substantial weight or has gained weight at a faster rate than previously, and is not taking relevant drugs is more likely to have an underlying disorder such as hypothyroidism or Cushing’s syndrome. All obese patients should have thyroid function tests performed on one occasion, and an overnight dexamethasone suppression test or 24-hour urine free cortisol if Cushing’s syndrome is suspected. Clinical assessment and investigations Assessment of the diverse complications of obesity requires a thorough history, examination and screening investigations. The impact of obesity on the patient’s life and work is a major consideration. Assessment of other cardiovascular risk factors is important. Blood pressure should be measured with a large cuff, if required . Associated type 2 diabetes and dyslipidaemia are detected by measuring blood glucose or HbA1c and a serum lipid profile, ideally in a fasting morning sample. Elevated serum transaminases occur in patients with non-alcoholic fatty liver disease Management The health risks of obesity are largely reversible. Lifestyle advice that lowers body weight and increases physical exercise reduces the incidence of type 2 diabetes. Most patients seeking assistance with obesity are motivated to lose weight but have attempted to do so previously without long-term success. Management Often weight will have oscillated between periods of successful weight loss and then regain of weight These patients may hold misconceptions that they have an underlying disease, inaccurate perceptions of their energy intake and expenditure, and an unrealistic view of the target weight that they would regard as a ‘success’. Management A reasonable goal for most patients is to lose 5–10% of body weight. The management plan will vary according to the severity of the obesity and the associated risk factors and complications. It will also be influenced by availability of resources. Management : Management : Lifestyle advice • Behavioural modification to avoid some of the effects of the ‘obesogenic’ environment is the cornerstone of long-term control of weight. Regular eating patterns and maximising physical activity are advised. • Where possible, this should be incorporated in the daily routine (e.g. walking rather than driving to work), since this is more likely to be sustained. • Alternative exercise (e.g. swimming) may be considered if musculoskeletal complications prevent walking. Management : Lifestyle advice Changes in eating behaviour (including food selection, portion size control, avoidance of snacking, regular meals to encourage satiety, and substitution of sugar with artificial sweeteners) should be discussed. Regular support from a dietitian or attendance at a weight loss group may be helpful. Weight loss diets In overweight people, adherence to the lifestyle advice may gradually induce weight loss. In obese patients, more active intervention is usually required to lose weight before conversion to the ‘weight maintenance ’ advice . Weight loss is highly variable, with patient compliance being the major determinant of success. There is some evidence that weight loss diets are most effective in their early weeks.